Heretofore, a continuous electric firing furnace is disclosed in Patent Document 1 in which an electric heater constituted of an electrically resistant heating element is arranged in the furnace to heat a material to be fired conveyed in the furnace. According to this document, it is disclosed that in a ceiling part of a furnace chamber, an electric heater such as a molybdenum disilicate-based cermet heating element is arranged in a direction crossing a conveyance direction of a treatment target at right angles. It is also disclosed that the heating element softens in excess of a temperature of 1300° C., and a heating part of the heater sags downwards, which might cause disconnection. Therefore, a support plate is arranged under the electric heater to prevent the sagging.
However, in recent years, a new material such as a functional ceramic or a highly heat-resistant and strong ceramic has been developed, and it becomes necessary to fire the material in an atmosphere controlled at a high temperature of 1500° C. At such a high temperature, even when an alumina-based refractory is used as the support plate to support the electric heater, the plate itself is unavoidably softened or deformed, which is a cause for shortening of life of the electric heater.
Furthermore, in a case where the material to be fired is an SiC-based ceramic containing an alkaline component or an Si component as a volatile component, problems occur that these components volatilized during the firing permeate and diffuse in the support plate of the alumina-based refractory and that the plate is more easily softened and deformed to further shorten the life of the support plate itself.
Patent Document 1: Japanese Patent Application Laid-Open No. 2000-310491: see claims, paragraphs from 0014, paragraphs of a mode for carrying out the present invention, FIG. 1 and FIG. 2